This invention relates to automatic transfer switches which are used to automatically transfer an electrical load from a normal source of electric power to an emergency source of electric power upon the happening of some predetermined event.
It is occasionally necessary to perform maintenance and repair work on an automatic transfer switch, or even to replace it. Therefore, a bypass switch is usually employed to provide continuity of power to the load while the transfer switch is out of service. Furthermore, it is important for safety reasons to completely disconnect or isolate the transfer switch from the power sources and load while work is being performed on the transfer switch. For this purpose, it is common to provide a separate isolation switch.
While systems of the type described above operate satisfactorily, they involve three separate switch devices (automatic transfer switch, bypass switch, and isolation switch), a considerable number of cable connections as well as long cable runs, and complicated safety interlocking schemes to prevent mishaps such as inadvertant disconnection of the load from a power source or connection of the load to both power sources at the same time. The complication and expense can be reduced by using a combination bypass and isolation switch as shown and described in U.S. Pat. No. 3,697,709.